O'Neill Cylinders are rotating megastructures where up to millions of people can live.
They typically come in pairs, each one counter-rotating each other to prevent negative gyroscopic effects.
These cylinders are typically lined with houses and cities within itself. The spin of the cylinder creates gravity with about the same intensity as Earth's. Up to 1 million people can comfortably live in the average-sized cylinder, with the largest ones able to contain up to a trillion people.
These cylinders are usually 32 kilometers long and have a radius of 5 kilometers. They spin at about 2.8 degrees per second, slow enough so that no human would be disoriented (however some alien species become very nauseous in a human-made O'Neill cylinder and require even larger ones with lower speeds).
At first these cylinders usually only have 50 percent of the atmospheric pressure of the Earth. This pressure allows for thinner walls although most humans experience extreme "altitude sickness" when exposed to these pressures of long periods of time, requiring over a month to acclimate.
With advancing technology, the pressure was able to increase to 90% which is similar to Denver, Colorado allowing for faster acclimation time.
Every O'Neill Cylinder has an area of zero-gravity in the center. This area typically has air at a lower pressure, however, some of the larger ones have a vacuum in the center. They are usually used for recreational activities and for transportation around the cylinder.